Looking for energy

The Department of Defense Research and Engineering has a prize offered to help solve the soldier’s battery problem. Soldiers are carrying more weight in batteries than in ammunition when going into battle these days. What the DDR&E is looking for could also solve a lot of RV battery needs. The goal from the Rules Document (.pdf):

Demonstrate a wearable electric power system providing 96 hours of equipment operation at less than half the current weight. The power system should attach to a garment (vest) and provide 20W average electric power for 96 hours with peak power requirements of up to 200W for short periods. All components, including the generation, storage, electronics, and connections must weigh 4kg or less, including the attachment system. The total minimum energy required is 1920 W-hr (20W * 96hr).

The typical 2 battery RV trailer has an energy capacity of about 2,000 W-hr and, at 33 kg for each battery, weighs nearly 70 kg. The usable energy is only about 1,000 W-hr before recharging is needed to obtain best results. That means the DDR&E is offering a prize to someone who can provide an energy source that weighs about a tenth of the batteries we currently use in our RV and provides perhaps double the energy.

During the 96 hours of operation the system must operate: (1) continuously (i.e., 24 hours a day); (2) at partial power; (3) for a number of periods of high power of up to 200W for 5 min; (4) in a non-air breathing mode in a sealed container for a time to be determined; (5) be attitude or vertical orientation and motion independent; (6) provide output voltages of 14V and 28V. The system must attach to a vest and operate while worn.

Of course, batteries aren’t the only source of energy. You’ve got those propane tanks on the front of your RV. Let’s see, 1 gallon of propane is 91,600 BTU’s of energy (gasoline has 125,000 per gallon, diesel 138,000, campfire wood 3,500 BTU/lb and household garbage about 2,500/lb). 1,000 W-hr is 3,412 BTU’s. That means that a gallon of fuel averaging about 100,000 BTU is about 30,000 W-hr. At 50% efficiency, that means a gallon would be about 7 times what is needed the DDR&E prize. That is about a half kilogram of fuel so the rest of the machinery could weigh only 3.5 kg. Compare that to your typical genset, like a Honda EU1000i, which weighs 15 kg. This weight does not include the oxygen needed from the air for fuel powered energy, which is probably about on par with the fuel weight.

Some of the key points, other than weight and capacity, in the prize offering include they peak power of 200w and the non-air breathing mode. The non-air breathing mode puts severe constraints on a fuel based system. The peak power requirement is also one that will make achieving the prize rather difficult as anyone who has tried to start an air conditioner with a 2kw genset knows.

The DDR&E probably isn’t too worried about cost at this point. They also didn’t invent their goals and requirements out of thin air either. They are pushing the envelope with a million dollar prize offer and just maybe some result will trickle down to something an RV enthusiast can afford and use.

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